linux-kernel - Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

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Date:   Tue, 21 Nov 2017 15:32:48 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, richard.fellner@...dent.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

On Tue, Nov 21, 2017 at 3:17 PM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 11/21/2017 02:46 PM, Andy Lutomirski wrote:
>>> GDT: R/O TSS: R/W at least because of trampoline stack entry code:
>>> EXEC+R/O exception stacks: R/W
>> Can you avoid code duplication by adding some logic right after the
>> kernel cpu_entry_area is set up to iterate page by page over the PTEs
>> in the cpu_entry_area for that CPU and just install exactly the same
>> PTEs into the kaiser table?  E.g. just call kaiser_add_mapping once
>> per page but with the parameters read out from the fixmap PTEs
>> instead of hard coded?
>
> Yes, we could do that.  But, what's the gain?  We end up removing
> effectively three (long) lines of code from three kaiser_add_mapping()
> calls.

I'm hoping we can remove kaiser_add_mapping() entirely.  Maybe that's
silly optimism.

>
> To do this, we need to special-case the kernel page table walker to deal
> with PTEs only since we can't just grab PMD or PUD flags and stick them
> in a PTE.  We would only be able to use this path when populating things
> that we know are 4k-mapped in the kernel.

I'm not sure I'm understanding the issue.  We'd promise to map the
cpu_entry_area without using large pages, but I'm not sure I know what
you're referring to.  The only issue I see is that we'd have to be
quite careful when tearing down the user tables to avoid freeing the
shared part.

>
> I guess the upside is that we don't open-code the permissions in the
> KAISER code that *have* to match the permissions that the kernel itself
> established.
>
> It also means that theoretically you could not touch the KAISER code the
> next time we expand the cpu entry area.

I definitely like that part.

--Andy